Abstract

The continued development of femtosecond mid-infrared (IR) sources with ultrabroad spectral width is critical for probing and controlling complex molecular structural dynamics on an ultrafast timescale. We report on a sub-20 fs, coherent mid-IR source with an octave-spanning spectral bandwidth (>2000cm−1) tunable from 2–8 micrometers (37.5–150 THz), with energy >0.4μJ/pulse at 1 kHz. The mid-IR pulses are generated by four-wave mixing during the filamentation of intense 800 nm and 400 nm pulses in various gas media. Spectral tunability is achieved by the choice of gas, pressure and input 800 nm pulse energy.

(a) BBIR spectra as a function of pressure for air. The inset shows the spectral tuning curves as a function of pressure for various gases. Spectra measured in Ne at 768 and 3740 Torr have respective center wavelengths of 5.0 and 4.5 μm. (b) Experimental XFROG trace of the BBIR pulse generated in air at 1000 Torr with (i) additional 5 mm of CaF2 in the beam path and (ii) the Ge LWP filter replaced with a 250 μm thick Si wafer. (c) Reconstructed electric field intensity and phase of the BBIR pulse. The minimum XFROG retrieval error is 0.0175 on a 256 grid. All the data in this figure is obtained with I800nm=3mJ/pulse.

BBIR pulse energy as a function of (a) the input 800 nm energy, (b) the ω/2ω time delay, and (c) the analyzer angle. The BBIR pulse is generated in air at 1000 Torr with I800nm=3mJ/pulse for (b) and (c).